Good question!
Here’s what GRRM has to say about powerful landed knights:
“As I see it, the title “lord” – when used formally, and not simply as an honorific –conveys not only prestige, but certain legal rights as well. The right of pit and gallows, as they were once called, for instance – i.e. authority to hang people and toss them into dungeons. A landed knight has rather less prestige – a lord outranks a knight at feasts and tourneys, for instance – and also somewhat lesser rights.But certain landed knights, of ancient houses, with extensive lands, and large strong castles, may be lords in all but name. These uber-knights may actually be more powerful than many smaller lordlings, so there’s an overlap. Their peculiar status if often reflected by taking a style that incorporates the name of their castle, such as the Knight of Ninestars.“
So I would guess that those “lords in all but name,” might well have a petty lord or two as part of their affinity, but it would probably be a very rare situation. House Templeton with their thousand swords, the Swyfts who are both landed knights and a principal house of the Westerlands, the numerous and powerful green-apple Fossoways, would probably be among those corner cases. But your Eustace Osgreys, your Lorches, your Cleganes, I doubt it.
As you and I were discussing, historical context probably matters quite a bit as to who would get vassal lords and who would not. House Glover, for instance, was a royal family in the days before the Starks totally consolidated power in the North, and it would have been a matter of course for them to have lords sworn to them; if they, like the Royces, were allowed to keep some or all of their royal bannermen, they might well still have lords sworn to Deepwood Motte, even if they themselves are only masters. The head of House Templeton or House Swyft, though a landed knight, is going to carry a lot more historical prestige with the family name – and potential to have vassal lords – than, as you point out, the head of House Clegane. If I’m right and the green-apple Fossoways became landed knights after the Third Blackfyre Rebellion – when a grateful crown, carving up the seats of attained Blackfyre reacher loyalists, gave Ser Raymun and his heirs a portion of that land – then maybe vassal lords came with New Barrel, in a feudal version of the “came with the frame family”.
Just remembered: the Glovers absolutely have vassals. The Woods, Boles, and Branches, and of course the Forresters. They might be a bit more “hill clannish” than your typical southern lord, given the way that Asha refers to them as clans, but in the North the clan chiefs are still considered petty lords.